High modulus polymer composites and methods of making the same

ABSTRACT

The invention provides methods of producing composite polymers by combining fillers with polymers in the presence of preformed high molecular weight polymer. Monomer polymerization can be initiated through the addition of initiators or by reactive chemical groups on the surface of the fibers. The composite materials formed possess superior mechanical properties compared to similar polymer composites made by either purely mechanical mixing or solely polymerization of monomers in the presence of the fillers.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority under 35 U.S.C. §119(e)from U.S. Provisional Patent Application No. 60/660,972 filed Mar. 11,2005, which is incorporated herein in its entirety by this reference.

GOVERNMENT INTEREST

This invention was made with government support under EnvironmentalProtection Agency (EPA)-grant RD-83153001-1. The government has certainrights in the invention.

FIELD OF THE INVENTION

The invention relates to polymer composites and more particularly, theinvention provides novel methods of making high molecular weight filledpolymeric matrices having desirable physical characteristics.

BACKGROUND OF THE INVENTION

The introduction of fibers and other fillers into a polymeric matrix isan established route to enhancing the physical properties of a chosenpolymer provided good dispersion and intimate interfacial adhesion canbe achieved. This method can also be very cost effective providing thefilling agents are obtainable at low to moderate cost.

Previous studies have shown successful improvements in mechanicalproperties of polymers by the formation of microcomposites throughpurely physical mixing of fiber fillers into the polymer. For example,Oksman et al. (Oksman, K., M. Skrifvars, and J. F. Selin, Natural fibresas reinforcement in polylactic acid (PLA) composites. Composites Scienceand Technology, 63(9): 1317-1324, 2003) embedded flax fibers into apolylactic acid (PLA) matrix and compared the resulting compositeproperties to polypropylene (PP) filled with the same fibers. It wasfound that the mechanical properties of the flax-PLA composites arepromising, since the composite strength was about 50% better compared tosimilar flax-PP composites that are used in many industrialapplications. However, microscopy studies suggested a lack ofinterfacial adhesion between the polymer matrix and the fiber surface.Additionally, Huda et al. (Huda, M. S., et al., Effect of processingconditions on the physico-mechanical properties of cellulose fiberreinforced poly(lactic acid). ANTEC 2004 Plastics: Annual TechnicalConference, Volume 2: Materials, 2:1614-1618, 2004; Huda, M. S., et al.Physico-mechanical properties of “Green” Composites from poly(lacticacid) and cellulose fibers, at GPEC, Detroit, USA, 2004) showedimprovement of the tensile strength, tensile modulus and impact strengthupon reinforcing PLA with cellulose fibers. However, the introduction ofcellulose fibers did not affect the glass transition temperaturesignificantly as measured by DSC.

Thus, there is a desire for improved polymer composites but there exitsa need for an improved method of making these filled polymers to achievethe desired polymer physical characteristics rapidly and at anacceptable cost.

SUMMARY OF THE INVENTION

The technology disclosed herein is a method of making polymercomposites. These methods are rapid and economically efficient ways toproduce polymer composites. The polymer composites produced by thesemethods have desirable physical characteristics. The polymer compositesproduced by these methods also have a homogenous or nearly homogenousdistribution of filler throughout.

The methods include the mixing of a monomer or oligomer with apre-formed polymer in the presence of a filler to initiate aninterchange reaction between a grafted layer of monomer or oligomer onthe surface of the filler and the pre-formed polymer, leading to theformation of a composite polymer.

In one embodiment, a method of forming a composite polymer includesmixing a filler simultaneously with a monomer that can react to form agrafted polymer layer on the surface of the filler and a pre-formedpolymer. In this process, an interchange reaction takes place between agrafted layer or monomer or oligomer on the surface of the filler andthe pre-formed polymer, to form a composite polymer.

In this reaction, the filler may be an organic filler such as woodfiber, wood flour, starch, straws, bagasse, coconut hull/fiber, cork,corn cob, corn stover, cotton, gilsonite, nutshell, nutshell-flour, ricehull, sisal, hemp or soybean.

Alternatively, the filler may be an inorganic filler such as a mineral,calcium carbonate, montmorilonite, kaolin, titanium dioxide, aluminatrihydrate, Wollastonite, talc, silica, quartz, barium sulfate, antimonyoxide, mica, magnesium hydroxide, calcium sulfate, feldspar, nephelinesyenite, microspheres, carbon black, glass, glass fibers, carbon fibers,metallic particles, magnetic particles, montmorillonite,buckminsterfullerene, carbon nanotubes, carbon nanoparticles, silicas,cellulosic nanofibers, synthetic silicates or synthetically preparednanoparticles. In a preferred embodiment, the filler is cellulose thathas been pre-treated with alkali prior to the mixing.

The monomer(s) used in these methods may be any monomer or oligomercapable of interacting with the filler, and particularly, interactingwith surface groups on the surface of the filler, and also capable ofparticipating in the interchange reaction with the chosen pre-formedpolymer. While these characteristics are necessarily dependent upon thechosen filler and pre-formed polymer to be used in these reactions,exemplary monomers include L-lactide, D-lactide, LD-lactide,caprolactone, caprolactam, ring opening monomers, ethyleneglycol-terphthalic acid, sebacoyl chloride—1,6 hexadiamine, stepreaction monomers, ethylene, propylene, styrene, methyl methacrylate,and vinyl monomers.

The pre-formed polymer(s) are similarly any polymer capable ofparticipating in the interchange reaction with the filler and themonomer. Exemplary polymers for use as the pre-formed polymer in theseprocesses include poly-lactic acid (PLA), polycaprolactone,poly(ethylene terephthalate), polyesters, polycaprolactam (Nylon 6),poly(hexamethyl sebacamide) (Nylon 6,10), polyamides, polyurethanes,polycarbonates, polyolefins, polyethylene, polybutadiene, polypropylene,polystyrene and polymethylmethacrylate.

In certain embodiments, catalysts may be used to increase the initiationrate or polymerization rate of the monomer(s) or to increase the rate ofthe interchange reactions or both. The polymerization and interchangereactions can be stopped through the use of a compound that deactivatesone or more catalysts used in these processes.

Exemplary catalyst of the interchange reactions include titanium(IV)isopropoxide (TIP), dibutyl tin oxide (DBTO), an alkyl tin(IV) compound,monobutyltin trichloride (BuSnCl₃), TBD(1,5,7-triazabiscyclo(4.4.0)dec-5-ene), acid catalysts, sulfonic andsulfuric acids, base catalysts, sodium methylate, sodium methoxide,potassium methoxide, sodium hydroxide and potassium hydroxide, organicbases, triethylamine, piperidine, 1,2,2,6,6-pentamethylpiperidine,pyridine, 2,6-di-tert-butylpiridine, 1,3-Disubstitutedtetrakis(fluoroalkyl)distannoxanes, 4-dimethyl-aminopyridine (DMAP) andguanidine, alkaline metal alkoxides and hydroxides, basic zeolites,cesium-exchanged NaX faujasites, mixed magnesium-aluminum oxides,magenesium oxide and barium hydroxide, 4-(dimethylamino)pyridine (DMAP),4-pyrolidinopyridine (PPY), salts of amino acids, and enzymetransesterification catalysts.

Examples of particularly preferred combinations of monomers andpre-formed polymers include (monomer/pre-formed polymer)lactide/polylactide, ethylene glycol-terphthalic acid/poly(ethyleneterphthal ate), ethylene/polyethylene, an ester/poly(ester), apolycarbonate/a poly(ester), and, a polyamide/a poly(ester).

In one embodiment, a method of mixing cellulose fibers with lactide andpolylactic acid to form a premix and then contacting this premix with acomposition including stannous octoate, triphenylphospine, and titaniumisopropoxide, to produce a composite polymer and then contacting thecomposite polymer with poly(acrylic acid) to form a stabilized compositepolymer is provided.

Additional embodiments include composite polymers made by mixing afiller with a monomer that can react to form a grafted polymer layer onthe surface of the filler and a pre-formed polymer to produce aninterchange reaction between a grafted layer on the surface of thefiller and the pre-formed polymer to form a composite polymer. In oneembodiment, this composite polymer is produced using a cellulose filler,lactide monomers and pre-formed poly-lactic acid (PLA).

This preferred polymer may be mixed in the presence of a catalyst suchas titanium(IV) isopropoxide (TIP), stannous octoate Sn(Oct)₂,triphenylphospine, and/or mixtures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a SEM micrograph showing untreated fibers (left) and thefibers after alkali treatment (right).

FIG. 2 is a chemical scheme for one embodiment of the present inventionusing a multifunctional initiator and the resulting polymer structure.

FIG. 3 shows the molecular structure of cellulose, exhibiting 6 hydroxylgroups available for initiating polylactic acid (PLA) polymerization asdescribed in the Examples appended below.

FIG. 4 is a graph of the molecular weight development as a function ofreaction time with and without the addition of PAA. (Experimentalconditions: 180° C., addition of PAA after 25 minutes reaction time).

FIG. 5 is a graph showing that the introduction of pre-formed polymerincreases the molecular weight (Experimental conditions: 200° C., 20minutes reaction time).

FIG. 6 is a graph of T_(g) versus molecular weight for samples withvarious composition of L- and D-lactide.

FIG. 7 is a graph of T_(g) versus fiber loading level for samplescontaining only pre-formed PLA (100/0 P/L) and for samples completelypolymerized in the presence of cellulose fibers (0/100 P/L) as describedin the appended Examples.

FIG. 8 is a graph showing the achievable crystallinity, as measured byDSC for samples containing only pre-formed PLA (100/0 P/L) and samplescompletely polymerized in the presence of cellulose fibers (0/100 P/L)as described in the appended Examples.

FIG. 9 includes graphs of the storage modulus versus fiber loading levelfor a) samples containing 100% pre-formed PLA, b) samples containing 65%preformed PLA and 35% lactide at the beginning of the reaction, c)samples containing 30% pre-formed PLA and 70% lactide at the beginningof the reaction, d) samples containing 100% lactide at the beginning ofthe reaction.

DETAILED DESCRIPTION OF THE INVENTION

The quality of polymer composites are influenced by various factors,including, principally 1) the aspect ratio of the filler particle (fiberor other geometry) used, 2) the particle orientation, 3) the volumefraction of filler, 4) the dispersion of particles in the polymericmatrix, and 5) the interfacial adhesion between the particle surface andthe surrounding matrix.

The interaction between the filler surface and the polymer matrix is anespecially important factor for the reinforcement potential of thefiller. In general, macroscopic reinforcing elements always containimperfections. Structural perfection is greater as the filler becomessmaller and smaller. However, the competing effect of increasingspecific surface area with decreasing size has to be considered.Additionally, in the case of a hydrophilic particle incorporated into ahydrophobic polymer, a lack of dispersion and interfacial adhesion isobserved. In order to improve these factors, reactive compatibilizationhas been used to mix the polymer monomers with the filler.Unfortunately, this process has been hampered by the time needed toproduce a composite polymer with an average high molecular weight andinsufficient mixing of the filler within the formed polymer.

The methods described herein overcome these problems with reactivecompatibilization resulting in composites with superior physicalcharacteristics to those formed by conventional means. The methods ofthe present invention include the mixing of a polymer filler with both alow molecular weight monomer or oligomer and a pre-formed high molecularweight polymer. The monomers polymerize to become chemically graftedonto the filler and undergo an interchange reaction with the pre-formedhigh molecular weight polymers thereby greatly decreasing the timeneeded to form a composite polymer. This reactive process also maintainsa high viscosity leading to greater stress within the mixture whichaides in dispersing the filler within the composite polymer.

The filler used in these processes can be any suitable filler that canreact with the monomer or oligomer. The polymer system chosen may be anycombination of polymers that are capable of participating in interchangereactions. One or more initiators or catalysts of the polymerizationreaction can be added to the reaction mixture. Additionally, one or morecatalysts for the interchange reaction can be added to the reactionmixture. Preferably, the chosen filler has a reactive surface capable ofinitiating the polymerization reaction of the monomers during mixing.When surface groups are employed as initiators for the polymerization,there are two major competing factors to be considered: 1) inpolymerization reactions, a high conversion rate of monomer is desiredand therefore a high number of initiating groups is advantageous, 2) toobtain good mechanical properties, a high molecular weight of thepolymer is required. High molecular weight implies a low number ofpolymer chains, corresponding to a low number of initiating groups.

The simultaneous introduction of the monomer and high molecular-weight,pre-formed polymer is able to address these competing factors. Themonomer polymerization is initiated from the reactive surface groups onthe fiber or particle surface. The presence of pre-formed highmolecular-weight polymer ensures a higher average molecular weight ofpolymer chains grafted to the fiber surface. The resulting materialconsists of high-molecular weight polymer chains grafted to the particlesurface in a polymer matrix. The resulting composites show improvedinterfacial adhesion and better dispersion properties. In addition,having high molecular weight polymer present initially means theviscosity is high so that more stress is transmitted during shearing ofthe sample, this leads to more effective dispersive mixing.

Examples of fillers useful in the methods of the present inventioninclude, but are not limited to, organic based fillers and combinationsthereof like, wood fiber, wood flour, starch, straws, bagasse, coconuthull/fiber, cork, corn cob and corn stover, cotton based fillers,gilsonite based fillers, nutshell and nutshell-flour based fillers, ricehull based fillers, sisal, hemp, and soybean based fillers. In addition,mineral fillers and combinations thereof may be employed including, butnot limited to, calcium carbonate, montmorilonite, kaolin and otherclay-based minerals, titanium dioxide, alumina trihydrate, Wollastonite,talc, silica, quartz, barium sulfate, antimony oxide, mica, magnesiumhydroxide, calcium sulfate, feldspar and nepheline syenite, varioustypes of microspheres (solid or hollow), carbon black, glass, glassfibers, carbon fibers, and various metallic or magnetic particles. Thisalso includes any nano-sized fillers and combinations thereof including,but not limited to, montmorillonite and other clay based particles,buckminsterfullerene, carbon nanotubes, other carbon basednanoparticles, silicas, glasses, cellulosic nanofibers, syntheticsilicates or any type of synthetically prepared nanoparticles. In oneembodiment in which an initiator other than the filler is added to thereaction, multifunctional initiators are preferred to further increasethe speed of the polymerization reaction and to encourage the formationof more highly-branched polymers.

The filler particles employed may be functionalized to enable reactionusing a variety of coupling agents. Such coupling agents include, butare not limited to, silane coupling agents, titanates, zirconates,aluminates, teolomers, and others.

The monomer and polymer molecules can be chosen from a wide variety ofknown polymers. Preferably, the polymerization of the monomers isinitiated in the presence of reactive groups on the surface of thefiller. Exemplary monomer/polymer combinations include, but are notlimited to, ring opening polymerization systems like lactide andpolylactide, caprolactone and polycaprolactone,caprolactam/polycaprolactam (Nylon 6), and numerous others. Also,condensation polymer systems can be utilized like ethyleneglycol-terphthalic acid/poly(ethylene terphthalate), sebacoylchloride—1,6 hexadiamine/poly(hexamethyl sebacamide) (Nylon 6,10), andnumerous others. Also vinyl polymer systems can be used including, butnot limited to, ethylene/polyethylene, propylene/polypropylene,styrene/polystyrene, methyl methacrylate/polymethylmethacrylate, andmany others. When polyester systems are utilized, the most importanttransreactions are alcoholysis, acidolysis, and transesterification.

Polycarbonates (PC) can also be reactively extruded with polyesters; themain reaction is the direct ester-carbonate exchange reaction.Polyamides (Nylons) can also be transreacted with polyesters or withthemselves; the relevant reactions include acidolysis, aminolysis, andamidolysis (Porter, R. S. and Wang, L. H., Compatibility andtransesterification in binary polymer blends, Polymer, 33:2019-2030;Groeninckx, G., Sarkissova, M. & Thomas, S. in Polymer Blends, Volume 1:Formulation (eds. Paul, D. R. & Bucknall, C. B.) 417-459 (John Wiley &Sons, New York, 2000). Other transreactions practicable under thepresent art include, but are not limited to, the transreaction ofchemical functionalities between amides-ethers, esters-lactams,esters-olefins, amide-olefins, and olefins-olefins.

As noted above, the reactions may optionally be conducted in thepresence of a catalyst that catalyzes the interchange reaction betweenthe growing grafted polymer chain on the surface of the filler and thepre-formed polymer. In the case of grafted polyesters, atransesterification catalyst can be used to further decrease the time offormation of a high average molecular weight composite polymer. Suitablecatalysts of the interchange reactions include titanium(IV) isopropoxide(TIP), dibutyl tin oxide (DBTO), Mono-, di-, and tetraalkyl tin(IV)compounds, monobutyltin trichloride (BuSnCl₃), TBD(1,5,7-triazabiscyclo(4.4.0)dec-5-ene), acid catalysts like sulfonic andsulfuric acids, base catalysts like sodium methylate, sodium methoxide,potassium methoxide, sodium hydroxide and potassium hydroxide, organicbases like triethylamine, piperidine, 1,2,2,6,6-pentamethylpiperidine,pyridine, 2,6-di-tert-butylpiridine, 1,3-disubstitutedtetrakis(fluoroalkyl)distannoxanes, 4-dimethyl-aminopyridine (DMAP) andguanidine, alkaline metal alkoxides and hydroxides, basic zeolites andrelated solid compounds such as cesium -exchanged NaX faujasites, mixedmagnesium-aluminum oxides, magenesium oxide and barium hydroxide,4-(dimethylamino)pyridine (DMAP), 4-pyrolidinopyridine (PPY), salts ofamino acids, and enzyme transesterification catalysts.

Post-reaction deactivation of the catalyst may be used to ensure acontrolled termination of the reaction and avoid further molecularweight changes during any subsequent molding and/or characterizationsteps.

The temperature at which the reaction is carried out is chosen to aid incontrolling the rate of reaction in order to minimize the time offormation for the desired composite polymer while allowing controlledpolymerization and interchange of the monomer and polymer molecules toallow for uniform distribution of the filler. Typically, the temperatureof the reaction is between about 20° C. and about 500° C. Preferably,the reaction temperature is between about 25° C. and about 400° C.

Mixing of the reagents during the polymerization and interchangereactions is typically applied through a controlled mechanical meanssuch as a commercial reactor, mixing device, or extruder, the extrudermay have a static mixer attached.

The composite polymers formed by the methods of the present inventionare preferably processed into a commercially desirable form such aspellets through cutting or grinding procedures after the polymerizationand interchange reactions have been terminated.

The novel methods of forming polymer composites of the present inventioneffectively increases the reinforcement of the material compared topurely mechanical mixing for the same mixing conditions. The propertiesof the polymer materials produced by the methods of the presentinvention become limited by a decrease in molecular weight as the fiberloading level increases due to an excess of initiating groups.Therefore, both the fiber loading level and the ratio of pre-formedmaterial to lactide at the beginning of the reaction are importantfactors in designing a commercial process based on this new approach.

Additional objects, advantages, and novel features of this inventionwill become apparent to those skilled in the art upon examination of thefollowing examples. Those of ordinary skill in the art will readilyunderstand that these examples are not limiting on the invention asdefined in the claims which follow.

EXAMPLES

The methods of the present invention are exemplified here by theformation of a polylactic acid (PLA)-cellulose composite polymer formedby an interchange reaction including transesterification of the lactideby the PLA in the presence of a transesterification catalyst.

L-lactide and pre-formed polylactide (PLA) were purchased from CargillDow Polymers, now NatureWorks, (Minnetonka, Minn.) and used withoutfurther purification. Lactide was dried under vacuum (22 inch Hg) at 50°C. for at least 8 hours prior to use. PLA was dried under vacuum (25inch Hg) at 80° C. for at least 14 hours before being processed.

The cellulose fibers used in this study (CreaTech TC 2500) were suppliedby CreaFill Fibers Corp (Chestertown, Md.). These fibers have an averagelength of 900 μm and an average width of 20 μm (L:D ratio=45). In orderto remove surface impurities and increase surface reproducibility, thefibers were pretreated according to the following procedure: 1) 10 gfibers were suspended in 500 ml aqueous solution of 8 wt % sodiumhydroxide (NaOH), 2) the solution was placed in a sonicator for 6 hoursat slightly elevated temperature (about 37° C.), 3) the fibers were thenfiltered and washed with distilled water until neutrality, 4) the washedand filtered fibers were dried in a convection oven at 70° C. for 24hours, 5) before being used in the preparation of composites, the fiberswere again dried under vacuum (25 inch Hg) at 80° C. for about 8 hours.Repeated washing with distilled water was found to be necessary in orderto remove any degradation products of cellulose during the basetreatment, since these by-products would cause the treated fibers tostick together and yellow upon drying. FIG. 1 shows the fibers beforeand after the pretreatment.

A special catalyst package was employed in the polymerization reaction.Commercially, PLA is polymerized from lactide that is produced from cornsugars by fermentation followed by reactive distillation. Variouscatalysts can be employed in the polymerization reaction. However, themost common catalyst used is the tin-compound stannous octoate Sn(Oct)₂.This catalyst requires hydroxyl groups as initiators, however, there isalways a competition between natural initiators (residual water,alcohols) present in the starting materials and added initiators(compounds added to the system containing hydroxyl groups in order toinitiate the reaction). Stannous octoate, Sn(Oct)₂, used in thesereactions was obtained from Sigma Aldrich and used as received. Themolar ratio, R, of lactide to stannous octoate for all reactions wasR=2500. The co-catalyst triphenylphospine, P(φ)₃, has a beneficialeffect on the polymerization kinetics of L-lactide in reactions withSn(Oct)₂ as catalyst. P(φ)₃ was purchased from Sigma Aldrich and addedwithout further purification in an equimolar amount to Sn(Oct)₂.Titanium(IV) isopropoxide (TIP) was used as transesterificiation agentin samples containing pre-formed polymer (at a level of 0.1 wt % ofPLA). For handling purposes, solutions of the catalyst and co-catalystas well as the transesterificiation agent were prepared in dry,distilled toluene.

Samples were mixed and polymerized in a Haake Rheomix 3000.Polymerizations were performed at 200° C. for 20 minutes at 100 rpm.After melting and premixing lactide with the pre-formed PLA, the fiberswere fed into the mixer. Once a reaction temperature of 200° C. wasreached, the required amount of solution containing the catalyst andco-catalyst was added. At the end of the reaction time, the requiredamount of PAA solution was added and mixed with the material for atleast a minute prior to extraction from the mixer. The catalyst wasdeactivated using poly(acrylic acid), PAA of a molecular weight of 2000g/mol, (at a level of 0.25 wt % of lactide) purchased from Sigma Aldrichand dissolved in dioxane for transfer purposes.

After preparation, the material was stored in a freezer for at least 3days prior to grinding in a Foremost 2A-4 grinder to a maximum particlesize of about 5mm with the vast majority of the pellets being about2-3mm in diameter. Samples for further testing were prepared by acombination of vacuum- and compression-molding. The material was firstmelted under vacuum (about 25 inch Hg) at 190° C. until the amount ofgas released decreased significantly. Afterwards the material wascompression molded at 180° C. for about 5 minutes under a load of 5000psi and then quenched between water-cooled plates.

Samples were analyzed for glass transition temperature (T_(g)) andamount of crystallinity in a Perkin Elmer DSC-7. The machine wascalibrated against an indium standard twice, and a baseline establishedon a daily basis. The DSC testing protocol was as follows: 1) heat from5° C. to 200° C. at 10° C./min, 2) hold at 200° C. for 5 minutes, 3)cool from 200° C. 4) heat from 5° C. to 200° C. at 10° C./min. T_(g) andamount of crystallinity were determined from data obtained on the secondheating cycle by inflection point method.

The glass transition temperature (T_(g)) of samples containing nocellulose fibers and polymerized from L-lactide corresponds to previousstudies that evaluated the influence of proportions of L- and D-lactideon the T_(g). The Fox-Flory plot for PLA, shown in FIG. 6, demonstratesthat the glass transition temperature increases with increasingmolecular weight before reaching a plateau at about 59° C. at amolecular weight of about 50000 g/mol. The materials prepared in thisstudy reach a molecular weight within the range of constant T_(g) for100% L-lactide.

The introduction of micro-sized cellulose fibers into a matrix ofpre-formed PLA does not significantly affect the glass transitiontemperature as measured by DSC as shown in FIG. 7. This observation isconsistent with previous studies. If no pre-formed PLA is present andthe material is completely polymerized in the presence of cellulosefibers, then the glass transition temperature decreases with increasingfiber loading level, as shown in FIG. 7. According to the Fox-Floryequation, a decrease in the glass transition temperature is anindication of lower molecular weight:${T_{g}\left( {\overset{\_}{M}}_{n} \right)} = {A - \frac{B}{\quad{\overset{\_}{M}}_{n}}}$A and B in the above equation are constants, with$A = {\lim_{\quad{{\overset{\_}{M}}_{n}\rightarrow\infty}}{T_{g}\left( {\overset{\_}{M}}_{n} \right)}}$

For 100% L-lactide PLA, values were determined to be: A=60.2° C.,B=-71.1° C. g/mol. Therefore, with increasing fiber loading level, lowermolecular weight PLA is produced due to an excess of initiating groups.

Achievable crystallinity was also determined by DSC and is shown versusthe fiber loading level in FIG. 8. For samples containing onlypre-formed PLA (100/0 P/L) no variation of the achievable crystallinitywas observed, although an increase would be expected if the fibers serveas nucleation agent. The reason for this observation is the presence oftalc in the pre-formed commercial material that already serves asnucleation agent. Samples completely polymerized from lactide (0/100P/L) show a significantly lower percentage of achievable crystallinityfor unfilled systems. As the fiber loading level increases, theachievable crystallinity increases for 0/100 P/L samples because thefibers can serve as nucleating agent.

Mechanical properties were determined by Dynamic Mechanical ThermalAnalysis (DTMA) using an ARES Rheometer (TA Instruments, PiscatawayN.J.) with torsional rectangular fixtures. Before testing, the machinewas calibrated for normal force and torque. Test conditions were 0.11%strain and 1 Hz. The thermal scanning was performed as follows: 1) heatfrom 30° C. to 110° C. at 20° C./min, 2) hold for 5 minutes at 110° C.,3) cool from 110° C. to 30° C. at 20° C./min, 4) heat from 30° C. to110° C. at 10° C./min (end condition: 30° C.). Moduli were determinedfrom data on the first heating run.

The measured values of the storage moduli G′ are summarized in Table 1,and plots of the results are shown in FIG. 9. The modulus increases whenpre-formed PLA is physically mixed with cellulose fibers (FIG. 9 a);these results are consistent with previous studies. In the systemcontaining 35% lactide, at the beginning of the reaction the modulusincreases with the fiber loading level up to 25% by weight of thepolymer mass; this is followed by a decrease in modulus for a furtherincrease in fiber loading to 35% (FIG. 9 b). This indicates a drop inmolecular weight at the highest fiber loading level due to excess ofinitiating groups. However, the highest achievable modulus in thissystem exceeds G′ achievable by purely mechanical mixing—there is a 43%increase with 35% fibers by physical mixing but a 50% increase with 25%fibers by reactive compatibilization. For systems containing 70% lactideat the beginning of the reaction (FIG. 9 c), a similar trend wasobserved; an initial increase in G′ with fiber loading level wasfollowed by a decrease beyond 25% fibers due to an excess amount ofinitiating groups resulting in low molecular weight PLA. Thereinforcement achievable in this system is better than in systems onlyrelying on physical mixing or systems containing 35% lactide at thestart of the reaction.

If the polymer is completely polymerized in the presence of the fibers,the modulus slightly increases for a fiber loading level of 15% followedby a sharp decrease at higher loadings (FIG. 9 d). The material obtainedwith 35% fibers did not allow mechanical testing due to very lowmolecular weight. Overall, comparison of the moduli measured at 40° C.shows that the best reinforcement for this lactide-PLA-cellulosecomposite polymer is achieved at 25% fiber loading level in the presenceof 30% pre-formed PLA. TABLE 1 Summary of storage modulus G′ formaterials tested. Fiber loading level Modulus G′ Reinforcement [wt %][GPa] [%] 100% preformed PLA, 0% lactide (100/0 P/L) 0 1.4 — 15 1.9 3625 1.9 36 35 2.0 43 65% preformed PLA, 35% lactide (65/35 P/L) 0 1.4 —15 1.7 21 25 2.1 50 35 1.5  7 30% preformed PLA, 70% lactide (30/70 P/L)0 1.5 — 15 1.8 20 25 2.3 53 35 2.0 33 0% preformed PLA, 100% lactide(0/100 P/L) 0 1.6 — 15 1.9 19 25 1.4 −22   35 — —

The molecular weight of materials without fillers was determined byintrinsic viscosity measurements. Solutions of the material wereprepared in tetrahydrofuran (THF) at concentrations of 0.005 g/ml andthe viscosity of the polymer solution measured in an Ubbelohdeviscometer at 30° C. The Schultz-Blaschke relationship allows asingle-point determination of the intrinsic viscosity [η] from viscositymeasurements of the pure solvent and the sample solution, given theSchultz-Blaschke coefficient k_(SB) is known for the givenpolymer-solvent system at a constant temperature.$\lbrack\eta\rbrack = \frac{\eta_{sp}}{c*\left( {1 + {k_{SB}*\eta_{sp}}} \right)}$

In the Schultz-Blaschke equation, where c is the solution concentrationin g/ml, k_(SB) the Schultz-Blaschke coefficient, and η_(sp) is thespecific viscosity defined by$\eta_{sp} = {{\frac{\eta}{\eta_{0}} - 1} = {\frac{t}{t_{0}} - 1}}$where t and t₀ is the time necessary for the solution and the solvent,respectively, to pass through a defined part of the viscometer. TheSchultz-Blaschke coefficient for PLA in THF at 30° C. was previouslydetermined to be 0.298±0.005.

Knowing the intrinsic viscosity, the viscosity average molecular weightcan be calculated by rearranging the Mark-Houwink equation$M_{V} = \left( \frac{\lbrack\eta\rbrack}{K} \right)^{\frac{1}{a}}$

The parameters in this equation, K and a, are also specific for thepolymer-solvent system at a given temperature.

The Stockmayer-Fixman equation allows the calculation of theweight-average molecular weight and appears as$\frac{\lbrack\eta\rbrack}{\quad{\overset{\_}{M}}_{W}^{1/2}} = {K_{\Theta} + {b*{\overset{\_}{M}}_{W}^{1/2}}}$

The factor K_(Θ) (Mark-Houwink constant for theta-conditions) is anexperimental quantity often used to quantify unperturbed polymer chaindimensions in dilute solutions. Values reported in the literature show alarge variability. However, careful experiments have determined thisconstant K_(Θ) to be 0.107±0.022 mL/g.

Post-reaction deactivation of the catalyst produced a controlledtermination of the reaction while avoiding further molecular weightchanges during molding and characterization measurements. Samples werecollected during the polymerization of PLA and the molecular weightdetermined as a function of time as a means of assessing theeffectiveness of polyacrylic acid (PAA) as a reaction-stopping reagent.As shown in FIG. 4, PAA is very effective in deactivating Sn(Oct)₂compared to a control sample. There is no further increase in molecularweight after the addition of PAA to the system, while the finalmolecular weight without PAA addition is not reached within an hourunder the present experimental conditions.

The average molecular weight of samples that are completely polymerizedin the Haake reaches about 77600 g/mol, whereas the pre-formed materialis of a molecular weight of 108100 g/mol after processing for 20 minutesat 200° C. in the presence of TIP. As expected, the introduction ofpre-formed polymer to the PLA synthesis leads to an increase in averagemolecular weight as depicted in FIG. 5.

The foregoing description of the present invention has been presentedfor purposes of illustration and description. Furthermore, thedescription is not intended to limit the invention to the form disclosedherein. Consequently, variations and modifications commensurate with theabove teachings, and the skill or knowledge of the relevant art, arewithin the scope of the present invention. The embodiment describedhereinabove is further intended to explain the best mode known. forpracticing the invention and to enable others skilled in the art toutilize the invention in such, or other, embodiments and with variousmodifications required by the particular applications or uses of thepresent invention. It is intended that the appended claims be construedto include alternative embodiments to the extent permitted by the priorart.

1. A method comprising: mixing a filler simultaneously with a monomerthat can react to form a grafted polymer layer on the surface of thefiller and a pre-formed polymer; wherein an interchange reaction takesplace between a grafted layer on the surface of the filler and thepre-formed polymer to form a composite polymer.
 2. The method of claim1, wherein the filler is an organic filler.
 3. The method of claim 2,wherein the filler is selected from the group consisting of wood fiber,wood flour, starch, straws, bagasse, coconut hull/fiber, cork, corn cob,corn stover, cotton, gilsonite, nutshell, nutshell-flour, rice hull,sisal, hemp, and soybean.
 4. The method of claim 1, wherein the filleris an inorganic filler.
 5. The method of claim 4, wherein the filler isselected from the group consisting of a mineral, calcium carbonate,montmorilonite, kaolin, titanium dioxide, alumina trihydrate,Wollastonite, talc, silica, quartz, barium sulfate, antimony oxide,mica, magnesium hydroxide, calcium sulfate, feldspar, nepheline syenite,microspheres, carbon black, glass, glass fibers, carbon fibers, metallicparticles, magnetic particles, montmorillonite, buckminsterfullerene,carbon nanotubes, carbon nanoparticles, silicas, cellulosic nanofibers,synthetic silicates, and synthetically prepared nanoparticles.
 6. Themethod of claim 1, wherein the filler is cellulose that has beenpretreated with alkali prior to the mixing.
 7. The method of claim 1,wherein the monomer is selected from the group consisting of L-lactide,D-lactide, LD-lactide, caprolactone, caprolactam, ring opening monomers,ethylene glycol-terphthalic acid, sebacoyl chloride—1,6 hexadiamine,step reaction monomers, ethylene, propylene, styrene, methylmethacrylate, and vinyl monomers.
 8. The method of claim 1, wherein thepre-formed polymer is selected from the group consisting of poly-lacticacid (PLA), polycaprolactone, poly(ethylene terephthalate), polyesters,polycaprolactam (Nylon 6), poly(hexamethyl sebacamide) (Nylon 6,10),polyamides, polyurethanes, polycarbonates, polyolefins, polyethylene,polybutadiene, polypropylene, polystyrene, and polymethylmethacrylate.9. The method of claim 1, wherein the mixing is conducted in thepresence of a catalyst that initiates or catalyzes the polymerizationreaction.
 10. The method of claim 1, wherein the mixing is conducted inthe presence of a catalyst that catalyzes the interchange reactionbetween the monomer and the pre-formed polymer on the surface of thefiller.
 11. The method of claims 9 or 10, further comprising stopping apolymer reaction by contacting the pre-formed polymer with a compoundthat deactivates the catalyst.
 12. The method of claim 10, wherein thecatalyst is selected from the group consisting of titanium(IV)isopropoxide (TIP), dibutyl tin oxide (DBTO), an alkyl tin(IV) compound,monobutyltin trichloride (BuSnCl₃), TBD(1,5,7-triazabiscyclo(4.4.0)dec-5-ene), acid catalysts, sulfonic andsulfuric acids, base catalysts, sodium methylate, sodium methoxide,potassium methoxide, sodium hydroxide and potassium hydroxide, organicbases, triethylamine, piperidine, 1,2,2,6,6-pentamethylpiperidine,pyridine, 2,6-di-tert-butylpiridine, 1,3-Disubstitutedtetrakis(fluoroalkyl)distannoxanes, 4-dimethyl-aminopyridine (DMAP) andguanidine, alkaline metal alkoxides and hydroxides, basic zeolites,cesium-exchanged NaX faujasites, mixed magnesium-aluminum oxides,magenesium oxide and barium hydroxide, 4-(dimethylamino)pyridine (DMAP),4-pyrolidinopyridine (PPY), salts of amino acids, and enzymetransesterification catalysts.
 13. The method of claim 1, wherein themonomer is lactide and the pre-formed polymer is polylactide.
 14. Themethod of claim 1, wherein the monomer is ethylene glycol-terphthalicacid and the pre-formed polymer is poly(ethylene terphthalate).
 15. Themethod of claim 1, wherein the monomer is ethylene and the pre-formedpolymer is polyethylene.
 16. The method of claim 1, wherein the monomeris an ester and the pre-formed polymer is a poly(ester).
 17. The methodof claim 1, wherein the monomer is a polycarbonate and the pre-formedpolymer is a poly(ester).
 18. The method of claim 1, wherein the monomeris a polyamide and the pre-formed polymer is a poly(ester).
 19. A methodcomprising: mixing cellulose fibers with lactide and polylactic acid toform a premix contacting the premix with a composition comprisingstannous octoate, triphenylphospine, and titanium isopropoxide, toproduce a composite polymer contacting the composite polymer withpoly(acrylic acid) to form a stabilized composite polymer.
 20. Acomposite polymer made by mixing a filler simultaneously with 1) amonomer that can react to form a grafted polymer layer on the surface ofthe filler and 2) a pre-formed polymer, wherein an interchange reactiontakes place between a grafted layer on the surface of the filler and thepre-formed polymer to form a composite polymer.
 21. The polymer of claim20, wherein the filler is cellulose, the monomer is lactide and thepre-formed polymer is poly-lactic acid (PLA).
 22. The polymer of claim21, wherein the mixing is conducted in the presence of a catalystselected from the group consisting of titanium(IV) isopropoxide (TIP),stannous octoate Sn(Oct)₂, triphenylphospine, and mixtures thereof.